Artificial Aggregates

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New Technologies to Produce Artificial Aggregates Have the Potential to Dramatically Mitigate Manmade Emissions of Carbon Dioxide and Help Close the Circular Economy Loop.

By Jonathan Rowland

Blue Planet’s synthetic limestone aggregate was specified for concrete poured as part of work at San Francisco International Airport. Source: Blue Planet.
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The Blue Planet process produces two product streams: a synthetic limestone (top) and an upcycled aggregate (bottom). Source: Blue Planet.
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Aggregate manufactured from bypass dust using ACT. Source: Carbon8 Systems.
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Carbon8 Systems’ CO2ntainer at a CRH cement plant in Mississauga. Source: Carbon8 Systems.

What connects the White Cliffs of Dover and your next lobster meal? The answer is calcium carbonate. Among the most common minerals in the earth’s crust, it is also the building block of that part of the lobster that turns dinner parties into sometimes messy affairs: the shell. Indeed, carbonate rocks – such as limestones – are formed from the shells of these crustacean delicacies, as well as corals and tiny coccolithophores and foraminifera, recrystalized and lithified over geological time. Carbonate rocks is also mined in vast quantities each year in almost every corner of the globe and is the most common aggregate used in concrete.

One aspect of calcium carbonate that may be overlooked, however, is its ability to permanently sequester carbon dioxide (CO2) in stable mineral form. It is the diffusion of CO2 into the earth’s oceans that provides lobsters and their arthropoidal cousins with the carbon they need to grow – through a process known as carbonate mineralization – their calcium carbonate (CaCO3) shells. This is the process that is reversed during the manufacture of cement, when CO2 is released during the thermal decomposition of limestone in the calciner.

A number of companies have now commercialized technologies that use mineral carbonation to remove CO2 from the flue gases of industrial plants to manufacture artificial calcium carbonate that can replace virgin limestone aggregate in construction. As 44% of the mass of calcium carbonate is CO2, one ton of limestone contains 440 kg of CO2 that is permanently sequestered in the crystalline state; the potential of this technology therefore is significant, explained Dr. Brent Constantz, founder and CEO of one such company, San Francisco-based Blue Planet Ltd.

“The whole concept is that you can make your rock locally and do it with material that you’re trying to mitigate – CO2,” Dr. Constantz told Rock Products. Not only does this reduce emissions of CO2, it limits the environmental impact from further open pit quarrying and long-distance transportation of mined aggregates. It also helps to build a circular economy, as the other input into such systems is construction or industrial waste that would otherwise end up in landfill.

Make Like a Lobster

The Blue Planet technology effectively mimics the biomineralization process used by lobsters to make their shells. CO2 from industrial flue gas (in the case of Blue Planet, the flue gas of a large gas-fired power plant in the San Francisco Bay area) is captured in an ammoniated water solution. It is combined with calcium sourced from demolished and returned concrete.

This concrete is bathed in an acidic solution to dissolve the cement and release calcium oxide.

Artificial limestone is then mineralized around a nucleus or substrate, sequestering the CO2 in a stable mineral form that can be used in a similar way to natural limestone. “We subject it to the same regular ASTM testing as any other geologic aggregate would be subjected to,” said Dr. Constantz. “But because it is a manufactured project, it is much more consistent that geologic limestone, which in most cases is highly variable. It can also be manufactured to specific size and density specifications, depending on the requirements of the project. For example, we supplied a specialized lightweight aggregate for work on Terminal 1 at San Francisco International Airport.”

When used in concrete, this consistency and ability to tailor to specific projects is of particular beneficial, allowing structural engineers to optimize the mix design. “Concrete is generally overdesigned with higher quantities of portland cement used to compensate for the natural variability of mined aggregate,” said Dr. Constantz. “Because the properties and quality of our aggregate is consistent, however, it provides the structural engineer much more confidence to engineer a mix design with lower portland cement content – reducing both the environmental footprint and the cost.”

In addition to construction project, Blue Planet’s product has the potential to be used in cement manufacture, which could be of specific benefit at plants that have been built in locations without local limestone resources or where the local limestone has been depleted. In this application, the CO2 released from the breakdown of the limestone is then used to create new limestone: only an additional source of calcium is required. The artificial limestone could also be designed to optimize the process – i.e. to sinter completely and at lower temperatures – and raises the possibility of producing a cement that is, at the very least, carbon neutral.

The second major advantage of the Blue Plant technology is recycling of aggregate from demolished and returned concrete. “After we have dissolved the cement to harvest its calcium, it liberates the sand and gravel that was inside the concrete, creating a second product stream beyond our artificial limestone. Instead of heading to landfill, this recycled aggregate is available for reuse,” explained Dr. Constantz. “When it goes through our process, the finer portion is dissolved, leaving a clean hard aggregate. This is much better for concrete production, as fines absorb more water, reducing workability and impacting hardening.”

The recycling of aggregate reduces the need for mining virgin aggregate, while also providing a key commercial benefit to Blue Planet. The revenue stream it creates is enough to service the debt on the project. As a result, Blue Planet’s internal rate of return is an impressive 24%, showing that carbon mitigation can be a profitable enterprise and not – as many assume – simply an added cost.

Closing the Circular Economy

In addition to capturing carbon, technologies such as Blue Planet’s go a significant way to closing the circular economy. A technology developed in the UK is also pioneering this approach. Called Accelerated Carbonation Technology (ACT), the process turns a variety of industrial residues and wastes into secondary aggregate. The range of waste materials ACT can be used to treat includes air pollution control residue (APCr, also known as flue gas treatment residue), ashes from the incineration of papermaking and wastewater sludges and biomass energy production, blast furnace slag and residual materials from cement manufacture, such as bypass dust.

ACT was originally developed from research undertaken at the University of Greenwich by Dr Colin Hills and Dr Paula Carey. In 2006, Carbon8 Systems, was spun out to commercialize this work. At the heart of the technology is the natural reaction of industrial residues with CO2 in the presence of water. This reaction usually happens very slowly, due to the low concentration of CO2 in the atmosphere. ACT – as the name suggests – speeds it up, reacting the materials in moist conditions with concentrated CO2 to form calcium carbonate, which can be used in a secondary process to form an aggregate suitable for use in the construction industry, as well as to manufacture a granulated fertilizer.

In 2010, a second company, O.C.O. Technologies (formerly Carbon8 Aggregates) was formed by the directors of Carbon8 Systems to develop the industrial-scale application of ACT in the UK for the commercial production of artificial aggregates. After investment by waste management specialist, Grundon Waste Management, O.C.O. Technology commissioned the first fully-commercial ACT plant at Brandon in Suffolk, UK, in 2012, with two further plants opened in 2016 near Bristol and 2018 in Leeds. Grundon Waste Management became the major shareholder in 2016 with the company officially rebranding as O.C.O. Technology in 2019.

O.C.O. Technology currently produces 350,000 metric tonnes per year of aggregate in the UK and has plans to increase this to over 750,000 metric tonnes within the next five years. The aggregates produced by the ACT process meet European standard EN 13055:2016 for lightweight aggregates and have been granted end-of-waste status in the UK. Marketed as carbon-negative aggregate, it is mostly used in the production of concrete blocks.

Going Global, Going Mobile

ACT has so far only been commercially deployed in O.C.O. Technology’s large-scale static facilities in the UK, which utilize CO2 produced as a by-product from other industries case, such as the manufacture fertilizer.

The company is however in the process of expanding internationally with two projects at an advanced stage: one in Asia and the other in Australasia. According to Dr. Peter Gunning, technical and quality manager at O.C.O. Technology, the first of these will be operational by mid-2020 with further projects in the USA and Europe also on the table.

“The potential for emission reduction is exciting and we are currently experiencing a lot of interest from international businesses looking to utilize ACT for this reason,” said the O.C.O. technical manager. “The scale of some of these projects is enormous with the potential for significant carbon capture and utilization.” As a key component of these projects, O.C.O. Technology is developing carbon capture technology, using lower-grade CO2 and flue gas.

Meanwhile, under the leadership of CEO Dr. Carey and Technical Director Dr. Hills, Carbon8 Systems has recently demonstrated a container-based solution, the CO2ntainer, that can be set up where the waste materials are produced, using CO2 from the flue gas of these industrial facilities.

“In autumn 2018, we demonstrated the first CO2ntainer at a CRH cement plant in Mississauga, Ontario,” Carbon8 System’s Strategy Director Ffion Rolph explained. “This successfully proved direct CO2 capture from flue gas, as well as the ability to make two marketable products: a lightweight aggregate and a mineral-rich fertilizer. In autumn 2019, we completed a similar project at Hanson’s Ketton cement plant in the UK and we expect to install five more CO2ntainers this year with customers in the cement, building materials and waste-to-energy industries”.

The CO2ntainer is designed to have a processing capacity that matches the amount of waste generated at an individual site (8,000-12,000 metric tpy), reducing the need to transport the material or CO2. The products from the process can be used in a variety of applications, principally in precast concrete blocks but also as pipe bedding, road sub-base, or higher value products, such as lightweight flooring, green roofing substrates or fertilizers, depending on the source of the original industrial residue.

The rise in interest in ACT has also been mirrored at Blue Planet: the Californian company has recently signed an agreement to develop Blue Planet systems at one hundred natural gas plants in North America. “We’ll also soon be announcing a relationship with a large multinational industrial company that will be doing the same thing on a more international basis, starting principally in Asia,” added Dr. Constantz.

A Game Changer?

Although only producing a small fraction of aggregates used every year, “recycled secondary aggregate is playing an increasingly important role in satisfying total demand and is promoted throughout the construction industry with many projects now having recycling and carbon reduction targets,” noted Dr. Gunning.

“Most aggregate is funded by some sort of public project, putting a lot of procurement power in the hands of governments,” added Dr. Constantz, CEO of Blue Planet. “In California, the Department of Transportation has a $12.5 billion budget and is one of the largest consumers of concrete. They could wield that procurement power to ensure carbon-sequestering materials are used in projects – and therefore have a tremendous impact mitigating CO2 and climate change without spending any more money than they already do. The same money that is currently being used to mine and transport virgin aggregates from British Colombia could be used to turn California’s CO2 into limestone for use in local construction.”

Technologies such as ACT and the Blue Planet process could therefore have the potential to play a significant role in reducing global CO2 emissions. Manmade CO2 emissions currently stand at about 40 billion metric tpy, according to the NOAA website.1 Meanwhile, global aggregate demand reached 42.3 billion metric tons in 2018, according to the latest statistics provided to Rock Products by The Freedonia Group, and likely to grow to 53.2 billion by 2028.2 The size of these numbers gives some idea of the opportunity presented by artificial aggregates. As Dr. Constantz concluded, “they get you from the base state up to a point where you can see how we could actually mitigate all anthropogenic CO2.”


1. Scott, M. and Linsdey, R., “Which emits more carbon dioxide: volcanoes or human activity”,, 15 June 2016, accessed at:

2. The Freedonia Group, Global Construction Aggregates, January 2020. For more information on this report:


My thanks to Dr. Brent Constantz of Blue Planet (, Dr. Peter Gunning of O.C.O. Technology ( and Ffion Rolph of Carbon8 Systems ( Further information about the companies and technologies mentioned in this article can be found on the company websites.